Abstract: The technology of steel structure strengthened with prestressed CFRP plates is an effective technique with a bright future． Prestress loss in the stage of tension and usage is an important problem in this technique． Based on the analysis and summary for the existing tension device and construction technology，using current analysis method of concrete structure strengthened with prestressed CFRP plates，separately calculate the prestress loss of CFRP plates used to strengthen steel structure and present the calculation method of every prestress loss． A set of independently developed tension and anchoring device was used to test 3 steel beams strengthened with prestressed CFRP plates． The experimental results show that: The proposed method is feasible，and the estimated value for prestress loss of steel structure strengthened with prestressed CFRP plates is suggested to be 15% tension control stress based on test．